A First-Principles Study on the Interaction between Alkyl Radicals and Graphene

The interaction between alkyl radicals and graphene was studied by means of dispersion-corrected density functional theory. The results indicate that isolated alkyl radicals are not likely to be attached onto perfect graphene. It was found that the covalent binding energies are low, and because of the large entropic contribution, ?${G{{{\ominus}\hfill \atop 298\hfill}}}$ is positive for methyl, ethyl, isopropyl, and tert-butyl radicals. Although the alkylation may proceed by moderate heating, the desorption barriers are low. For the removal of the methyl and tert-butyl radicals covalently bonded to graphene, 15.3 and 2.4 kcal?mol-1 are needed, respectively. When alkyl radicals are agglomerated, the binding energies are increased. For the addition in the ortho position and on opposite sides of the sheet, the grapheneCH3 binding energy is increased by 20 kcal?mol-1, whereas for the para addition on the same side of the sheet, the increment is 9.4 kcal?mol-1. In both cases, the agglomeration turns the ?${G{{{\ominus}\hfill \atop 298\hfill}}}$<0. For the ethyl radical, the ortho addition on opposite sides of the sheet has a negative ?${G{{{\ominus}\hfill \atop 298\hfill}}}$, whereas for isopropyl and tert-butyl radicals the reactions are endergonic. The attachment of the four alkyl radicals under consideration onto the zigzag edges is exergonic. The noncovalent adsorption energies computed for ethyl, isopropyl, and tert-butyl radicals are significantly larger than the graphenealkyl-radical covalent binding energies. Thus, physisorption is favored over chemisorption. As for the ?${G{{{\ominus}\hfill \atop 298\hfill}}}$ for the adsorption of isolated alkyl radicals, only the tert-butyl radical is likely to be exergonic. For the phenalenyl radical we were not able to locate a local minimum for the chemisorbed structure since it moves to the physisorbed structure. An important conclusion of this work is that the consideration of entropic effects is essential to investigate the interaction between graphene and free radicals.